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This enables a TTY and console on the MIPS EJTAG Fast Debug Channels,
if they are present. This can be useful when working with an EJTAG
probe which supports it, to get console output and a login prompt via
EJTAG without needing to connect a serial cable.

TTY devices are named e.g. ttyFDC3c2 (for FDC channel 2 of the FDC on
CPU3).

The console can be enabled with console=fdc1 (for FDC channel 1 on all
CPUs). Do not use the console unless there is a debug probe attached
to drain the FDC TX FIFO.